Solid-state imaging devices including a CMOS sensor are currently used for various purposes such as a digital still camera, a video movie, and a monitoring camera. The following imaging properties are requested for the application to a digital camera, a video movie, a monitoring camera, and the like. In other words, image capture can be performed at a high S/N ratio when a dark object is imaged, and further the image output resolution is provided also when a sufficiently bright object is imaged. In this manner, if the S/N ratio at the time of imaging a dark object is excellent, and also a bright object can be imaged, there are advantages that an image with what is called a wide dynamic range can be captured and that natural reproduction similar to one through the human eyes is achievable. However, in recent years, a request to reduce the size of an imaging optical system is strong. On the other hand, a request of high resolution is also increasing at the same time There are tendencies to reduce the pixel size. Therefore, it is becoming difficult to obtain such an image with a wide dynamic range as above. The circumstances are described below.
In other words, if the area of each pixel is reduced, then the area of a photodiode that accumulates signal charge generated by photoelectric conversion in the pixel is also reduced accordingly at the same time. However, the upper limit of the amount of signal charge that can be stored in the photodiode is roughly corresponding to the area of the photodiode. Accordingly, if the area of the pixel is reduced, the upper limit of the number of signal charge that can be stored in the photodiode, that is, the number of saturation electrons is reduced at the same time. In this case, image information of signals above the number of saturation electrons cannot be obtained. Accordingly, there arises a limit to the brightness at which a object can be imaged. Hence, it becomes difficult to obtain an image with a wide dynamic range.
Furthermore, when the area of each pixel is reduced, the following problem can occur: In other words, if the area of a pixel is reduced, the size of a MOS transistor configuring an output circuit placed in the pixel is also reduced at the same time. However, if the size of the MOS transistor configuring the output circuit of the pixel, for example, an amplifier transistor configuring a source follower circuit to serve as the output circuit, is reduced, 1/f noise or RTA (Random Telegraph Signal) noise occurring in there is increased. If so, when a dark object is being imaged and the amount of signal charge is small, the S/N ratio against the noise is reduced. Accordingly, in this case, a playback image may become a low quality image with much noise.
Furthermore, when the area of each pixel is reduced, the following problem can occur: In other words, if the area of each pixel is reduced, light incident on a pixel tends to leak into an adjacent pixel. However, a color filter to acquire a different color signal is generally placed in the adjacent pixel. Accordingly, in this case, color mixing may occur severely. Moreover, even if incident light does riot leak out to the adjacent pixel, when photoelectrons are generated in a part close to the boundary between pixels within a semiconductor region configuring a pixel, the photoelectrons leak into an adjacent pixel due to thermal diffusion or the like. Accordingly, color mixing may be increased due to a similar reason. If the degree of color mixing is high, color reproducibility reduces. Accordingly, it becomes difficult to obtain an image with high saturation on a playback screen.